This invention relates to heated platens in general and more particularly to an improved heated platen less subject to leaks. Heated platens are used for various functions such as for example the expansion of thermo formable material. Typically platens of this nature are made of aluminum and contain within them a plurality of parallel channels through which a heat exchange fluid may be passed in order to heat the platen. In large platens, the heat exchange passages are divided up into zones so that all areas of the platen are equally heated, otherwise the fluid, by the time it went from end of the platen to the other, would be considerably cooled and there would be uneven heating.
One type of heated platen that is of particular interest if a platen used in carrying out the expansion of a thermoformable material using a process such as that described in U.S. Pat. No. 3,765,810 and as also described in U.S. Pat. No. 4,113,909 granted to Donald R. Beasly and assigned to the Norfield Corporation, the same assignee as the present invention. In the process disclosed therein, a thermoformable plastic material is disposed between two platens, each of the platens having a surface containing a large number of vented openings. The platens are heated to a temperature of equal to or greater than 70.degree. C. at which point the thermoformable material bonds by hot tack adhesion to the platens at areas other than those containing openings. The platens are moved apart and venting occurs at the openings. Thus, as the platens are pulled apart a cellular structure results as more fully disclosed in the aformentioned patents. The platens are then cooled whereupon the expanded material can be removed from between the platens.
The type of platens which have been used commercially for this purpose contain a plurality of circular holes all of which are vented, the holes being arranged in rows and columns which are staggered. The aformentioned U.S. Pat. No. 4,113,909 describes the manner in which such platens can be supported so as to maintain a planar surface. Although the method and apparatus described therein has been used in commercial applications to produce large panels of for example 4.times.8 feet, various problems have occured with platens.
At the vented openings, vent passages have been previously drilled through the platens in a direction perpendicular to the plane surface of the platen. Only small holes can be used so as to avoid running into the channels traversing the platen which contain the heat exchange fluid. Because of the small diameter of the vent passages and the materials used in the process, the vent passages tended to become clogged. A further problem with platens containing perpendicularly disposed vent passages with apertures on the top surface of the platens, is that cracks have formed in the area between the heat exchange channel and the holes. Such cracks are not accessible from the exterior of the platens, as are cracks which might occur in the bottom of the platen. Thus, the occurance of such cracks, may mean that the whole platen has to be scrapped and a new one made, since remedial measures such as welding are not possible.
A further problem has been that of leaks, in particular leaks at the edges of the platens and also at the top or bottom of the platen. In the previously used platens a plurality of bores are formed extending across the width of the platen parallel to each other for the induction of the heat exchange fluid. These are divided up into zones with five bores or channels forming a zone. The heat exchange fluid is supplied, from below to the first of these channels at one side of the platen. It is removed from the fifth channel at the other side of the platen. In order to permit a back and forth flow, the ends of the platens were milled between bores to form a continuous flow channel. Upon completion of milling, an end plate was placed over the ends of the continuous flow channels and welded to the sides of the platen to cover up the exposed channels, thereby forming closed flow paths. Thus, one end plate covered the various zones on the side of the platen. In practice, a number of problems have resulted. First, cracks have developed at the end plate, or between the end plate and the platen, resulting in leaks. Furthermore, seepage of the heat exchange fluid from one zone to another has occurred reducing its effectiveness. These problems are found not only in platens used in an expansion process as described above but in other heated platens, particularly aluminum platens subjected to thermal shock.
It is the object of the present invention to provide a platen which avoids the problems of leakage at the end plates, including leakage to the outside and leakage from one zone to the other.
It is a further object of the present invention to avoid, in a platen containing a plurality of vent passages in its surface, cracking in the area of the vent passages.
In accordance with the present invention, the first object noted above, is achieved by utilizing separate end plates for each zone, which end plates are welded in place over a respective zone. This gives a number of advantages, in particular, it prevents the possibility of leakage from zone to zone. More significantly, however, by using a plurality of smaller end plates it avoids large stresses which are present when a single continuous end plate is used, reducing the possibility of cracks. In accordance with the present invention, it is preferred that the bore containing the inlet on one side of a particular zone not be covered by the plate, but rather have inserted therein a plug which is then welded to the platen. An end plate is then placed over the four remaining bores, after connecting channels have been milled between adjacent bores. Finally, adjacent plates and the welded plug are welded with a fillet weld to further strengthen and seal the arrangement.
In a plate of the type containing horizontal vent passages in its surface, in order to avoid cracks near that surface, the milled connecting channel between adjacent bores, or heat exchange fluid channels is milled such that its boundary is no closer to any openings, in particular to vent passages or vented openings than are heat exchange channels extending across the platen. What this means is that, rather than milling a channel having the same width as a diameter of the transfers heat exchange fluid channels, a connecting channel with a lesser width and greater depth is milled at the side of the channels furthest from the vented openings. In this way, the distance from any vent passage to the continuous flow channel is increased so that there is a greater amount of metal present, thereby materially decreasing the chance of cracks.